Position-measuring devices are needed in many different technical fields in order to determine the position (length or/and angle) of movable components in installations and machines. Such position-measuring devices are divided into two groups based on their operating principle. First of all, there are incremental, position-measuring devices in which the position is ascertained based on the counting of graduation periods of an incremental graduation, and secondly, there are absolute position-measuring devices in which the position is obtained by scanning and evaluating an absolute graduation.
Compared to absolute position-measuring devices, incremental position-measuring devices have a simple, robust design, but have the disadvantage that immediately after being switched on, no position information is available, and a reference mark must first be overtraveled by what is referred to as a reference measurement, in order to be able to infer the absolute position. That is why in many technical sectors, absolute position-measuring devices are employed by preference, using which, an absolute position value is available at any time, even immediately after being switched on For example, an absolute position-measuring device is described in European Published Patent Application No. 0 660 209.
One technical field in which the use of absolute position-measuring devices is still problematic involves installations or machines which are exposed to ionizing, high-energy radiation or whose field of application requires the use of such radiation. One such field is the medical technology field, where ionizing, high-energy radiation is used in targeted fashion to cure diseases or to slow down their progression. Predominantly gamma radiation, X-rays or particle radiation (protons, neutrons, electrons, etc.) are used.
Due to their simple structure, incremental position-measuring devices which are exposed to such radiation behave quite robustly. Absolute position-measuring devices, on the other hand, which require a more complex design in order to ascertain an absolute position value, are inclined to break down when they are exposed to ionizing, high-energy radiation.
If an absolute position-measuring device is nevertheless to be used in such an environment, e.g., in a device of medical technology for the irradiation of tumor tissue, two design approaches tend to be used: first of all, shielding the absolute position-measuring device from the harmful radiation, and secondly, constructing the position-measuring device from what are referred to as radiation-hardened components, e.g., components which are adapted especially for this application area.
Both approaches are unsatisfactory in practice. In order to attain sufficient shielding effect, a centimeter-thick lead casing is necessary, which is undesirable chiefly because of its weight, but also because of its significant space requirement. Compared to standard components, radiation-hardened components are usually extremely expensive and are only available in large housings. Therefore, to construct an absolute position-measuring device from radiation-hardened components would result in a large construction and a very high price. In addition, not all components needed or desired are available in radiation-hardened form.